1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for automatically switching off seismic equipment used for marine geophysical exploration when a given depth is exceeded.
2. Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled seismic source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, a seismic source is towed by a vessel at a certain depth along direction X as illustrated in FIG. 1. The acoustic waves from the seismic source (air gun, water gun, vibratory source, etc.) propagate in all directions. FIG. 1 shows a source array 104 being towed behind a vessel 101 at a shallow depth. The source array 104 may be connected to a float 102 for floating at a given depth and also to the vessel 101 by an umbilical 111 for receiving, for example, electrical signals and/or compressed air. When the source array is activated, acoustic energy is coupled into the water and transmitted into the earth, where part of the energy is partially reflected back from the ocean bottom 113 and from rock formation interfaces 112 (rock layer that has a change in acoustic impedance). Sensors or receivers 106, located along a streamer 105, are used to record the reflected energy. Such receivers may include hydrophones, geophones and/or accelerometers. The receivers can be encapsulated in either fluid-filled or solid streamers 105 that are also towed by vessels at shallow depth.
Currently, it is typical for one vessel to tow multiple streamers with diverters employed to ensure streamer separation by a fixed distance. In order to maintain the proper spacing between the streamers and sources, the vessel moves forward continuously, typically at a rate of about 4 knots (2 m/s). In some cases, the positions of the streamers can be controlled so that all receivers are at a common depth, or in other cases the receivers in each streamer are controlled to follow a particular depth profile.
Modern streamers are equipped with birds, compasses and GPS receiver buoys. Birds are devices equipped with wings or fins, and the birds are spaced at intervals and are in communication with the vessel to control streamer depth and transverse spatial position. Alternatively, the receivers can be stationary and positioned on the ocean floor as autonomous nodes or in an ocean bottom cable.
Depending upon the sensor type, the returning energy is recorded as a pressure, velocity or acceleration variation as a function of time at each receiver position. Combining recordings made at multiple source and receiver locations can be used to form an image of the subterranean features of the earth. Images formed from reflection seismology are useful for locating structures that are indicative of oil and/or gas reservoirs.
However, the above-noted technology may be used not only for determining an image of the subsurface, but also objects present in the water, e.g., whales, dolphins, submarines, etc. Because of this dual use of the seismic acquisition systems, and in order to prevent commercial seismic systems to be used in the military field, there are national and international regulations requiring that each hydrophone be provided with a pressure-sensitive switch that switches off the hydrophone if it achieves a depth greater than 35 m.
Effective design of such a pressure-sensitive switch to meet these requirements has not been easy. It is desirable that such a switch be inexpensive, particularly because seismic survey operations today use a number of streamers, each of which may have hundreds of hydrophones that must each have a depth-limiting device. The pressure at which the switch closes cannot deviate substantially from the desired depth setting, because the hydrophones operate in large arrays, and the premature disabling of some of the hydrophones can degrade the array's performance or even require suspension of a survey, at great expense.
De Groot et al., in U.S. Pat. No. 6,318,497, the entire content of which is incorporated herein by reference, discloses a pressure-sensitive switch including a base member having a mounting surface formed of electrically-insulating material. Two electrodes are mounted on the base member, with each electrode having an electrically-conductive contact surface. A flexible, corrugated diaphragm has its periphery secured in a fluid-tight manner to the mounting surface with its electrically-conductive central portion overlying the contact surfaces of the electrodes. The plural corrugations are exposed to the external pressure. When the switch is exposed to atmospheric pressure, the central portion of the diaphragm does not touch the contact surfaces of said electrodes, but when the switch is exposed to a pressure greater than the atmospheric pressure, this central portion is forced into contact with both contact surfaces, thus electrically connecting the two electrodes.
The switch disclosed by De Groot et al. has certain drawbacks. The corrugated diaphragm is unnecessarily complex, requiring a high degree of exacting quality control, which increases the cost of the entire unit. Also, the corrugated diaphragm must be very carefully set. Furthermore, slight variations in the configuration of the corrugated diaphragm result in the failure of the electrically-conductive elements of the switch to make contact.
Thus, there remains a need for a simple, inexpensive, lightweight, pressure-sensitive switch for mounting in a hydrophone cable, and other applications, which eliminates the corrugated diaphragm. The present depth-limiting switch is directed to filling these and other needs in the art.